MNTOR-4499: subscriber id foreign key cascade for feature_flag_events

[mansaj]

References:

Jira: MNTOR-4499

Description

Add cascade update/del for feature_flag_events fk

Checklist (Definition of Done)

• [x] Localization strings (if needed) have been added.
• [x] Commits in this PR are minimal and have descriptive commit messages.
• [x] I've added or updated the relevant sections in readme and/or code comments
• [x] Product Owner accepted the User Story (demo of functionality completed) or waived the privilege.
• [x] All acceptance criteria are met.
• [x] Jira ticket has been updated (if needed) to match changes made during the development process.
• [x] Jira ticket has been updated (if needed) with suggestions for QA when this PR is deployed to stage.

[github-actions[bot]]

Preview URL :rocket: : https://blurts-server-pr-5886-mgjlpikfea-uk.a.run.app

[Vinnl]

Closing this, since I'm fairly sure we don't want to delete audit events. There are two other PRs option with other options, with this one I think being preferred: https://github.com/mozilla/blurts-server/pull/5905